Why is this an issue?

The font-family (and the shorthand font) CSS property specifies a prioritized list of one or more font family names and/or generic family names for the selected element.

If none of the font names defined in a font or font-family declaration are available on the browser of the user, the browser will display the text using its default font. It’s recommended to always define a generic font family for each declaration of font or font-family to get a less degraded situation than relying on the default browser font. This lets the browser select an acceptable fallback font when necessary.

The list of generic font families is as follows:

How to fix it

You should always include at least one generic family name in a font-family list, since there’s no guarantee that any given font is available.

Code examples

Noncompliant code example

a {
  font-family: Helvetica, Arial, Verdana, Tahoma; /* Noncompliant; there is no generic font family in the list */
}

Compliant solution

a {
  font-family: Helvetica, Arial, Verdana, Tahoma, sans-serif;
}

Resources

Documentation